Ambient occlusion in Principled BSDF
You don't connect it at all. The lack of an AO input on Cycles' Principled BSDF node is not an oversight. See my answer here about what AO is and is not for: Adding Ambient Occlusion to Material

Ambient occlusion is so-named because it is occlusion for an "ambient" light type. Cycles does not really have an ambient light, at least not one that accepts pre-cache occlusion data. (the world AO effect is technically an ambient lamp, but it is the only one, and as of 2.79 it always samples its own occlusion on the fly, never from a texture). Since there is no ambient light, shaders have no need to define shading for it.

If you want to use AO data for mesh-dependent texturing effects, you should do these while authoring the texture, that way you do not need to load a separate AO map at render time.